what year 12a is best?

Dave do you equate more ignition timing into more heat therefor less hp? and could this be offset with a lower running temp?

[Dave Damouth]

Which Dave, the smart one or me? I'd say slower burn due to combustion chamber shape. Slower burn is not necessarily bad, but if the real Dave says a few less hp, that is bad. Unless the manifold is affecting exhaust port flow differently depending on the combustion pocket. I'm gonna stop now, this is making my head hurt...

[Mazdatrix]

Never looked into any way to "measure" it. My guess was more required timing = less efficient combustion, which we then attributed to the shape and irregularities of the pockets on the 83-85 rotors.

If you look at a bunch of these 12A rotors, the 83-85's are normally a pretty bad blend of the pocket casting edge into the machined face of the rotor, where the '79 (actually '76)- '82's have a much more consistant pocket shape and blend into the face.

Temperature wise, my dyno keeps the water at a solid 190 degrees, and I usually keep the oil under 210. (Outside water supply, fans, spray bars etc.) (actually, the water spray onto oil coolers is misting stuff right out of Home Depot). All race engines that leave here are 1700+ exhaust temperature.

Btw, I promised myself that the next Pro7 engine we dyno, I would make a movie of it and post it on my www.mazdatrixmovies.com site. It is SO cool when you can SEE the exhaust gases INSIDE the cast iron manifold!!

thanks Dave, as I said I have all possible combos so I guess ill go with 79 rotors and housings with 81 fly and counter and leave the 84's for the E/P motor and lighten them down to 2 pounds, hehe

[RacinRich]

Mazdatrix,
Could you give any advice/tips on a simple procedure to optimize timing using a chassis dyno? Also what about tuning jets? Please consider that some of us don't own our own dyno and can only buy "runs" at a chassis dyno ($$$). You've written several times that optimum timing and jet tuning are very specific to each engine and now I'm worried that I've been missing some extra HP.

[Mazdatrix]

What class, engine, mods, exhaust do you have?

[RacinRich]

ITA/7 with legally blueprinted/balanced 12A, Racing Beat header with dual pipes back to axle, then one 2 1/2" over axle into muffler. One MSD with direct fire coils.

[Mazdatrix]

"Tuning" --
I have not done full dyno testing on an ITA engine (nobody ever wanted to pay for dyno time) -- so I can only "point" you in the direction we saw with the Pro7 engines (NO header, just the stock cast iron manifold).

We ALWAYS run with an EGT gauge !! Anything above about 1740F and you are either too lean OR too rich !! (Yes, you do not know until you change the jets). If I see it heading toward 1800, I stop and change something.

--- This is ALL Pro7 stuff here -----

Timing wise - I start with 25 lead and 13 trail. Then you MUST do carb jetting before timing.

Try 125 primary and 155 secondary, then lean the secondary out till you no longer gain power, then go back up one step.
Then try the primary down one (120) and see if the gain on the top end is worth the loss of power on the bottom.
Etc Etc Etc Etc Etc -----

These can be VERY lean settings, and it is not hard to kill the engine while you are trying to "find the ballpark!!"

AFTER the jetting is optimized,THEN you play with timing.
Try about 16-18 degrees trail.
Put trail back at 13, and try about 28 lead. If it likes that, then try decreasing the split.
Etc Etc Etc Etc ----

On a chassis dyno, watch out you do not set the floor of the car on fire (NOT joking!!). Watch the paint, any undercoating, etc, anywhere near where the exhaust runs.

Dave ---

[dfm]

dyno time
how much is dyno time Dave?

[Mazdatrix]

Engine dyno is $500/day.

Sounds expensive, but more changes can be done in one day than 4-10 race weekends.

Every one I do, I think about doubling the price when I look at how long it really takes me.

Has to be scheduled WAY ahead - MANY restrictions:
Can't run it during working hours, after about 9:30, Thursday nights (farmers market behind me), many weekend afternoons and evenings if they are having a concert in the STUPID concert area they built behind me!!, anytime the condos 2 blocks away call the cops about "that loud chain saw place again", etc etc.

Rotary ONLY, doubt any OEM fuel injection would be cost effective - WAY too many sensors, boxes, and wires to pull out of the car. It would be most of a full day just to wire/hook everything up.

Break-in is NOT included !!! I break-in ALL race motors AT LEAST 6 hours with variable load. Rough "break-in" cost on a fresh engine is maybe $300 ??
Costs are human time only - extras are gas (a LOT), jets, plugs, filters, gaskets, etc.

I normally do not charge any break-in time on engines we build. A Pro7 engine (and probably ITA) would normally be done in max of one day and it's easiest to get it set up and ready to run on a Friday night, then dyno on Saturday.

I supply the ear plugs, and I encourage the customer to come and help (but I DO make you work). I find it very good for the customer to see what happens with various changes, and know why the changes are made.

Dave